It first appeared in Feng Menglong's Yu Shi Ming Yan, Volume XIII, Zhang Daoling's Seven Examinations of Zhao Sheng in Ming Dynasty.
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In modern times, the original phrase "the flower falls with the flowing water intentionally, and the flowing water has no love for it" has been changed to "the flower falls with the flowing water intentionally, and the flowing water has no love for it".
"The fallen petal intentionally follows the flowing water, and the flowing water has no beloved fallen petal" comes from a seven-character quatrain.
The full text is: I turn my heart to the bright moon, and the bright moon shines on the ditch. Flowers fall with the flowing water intentionally, but the flowing water does not fall unintentionally.
It means: I am good to you, but you are cold and ungrateful. My sincere efforts have not been rewarded and respected.
This seven-character quatrain is from the leaf-sweeping record of Chronology of Qing Poetry, which is a large-scale chronicle document of Qing Poetry edited by Qian Zhonglian, a famous expert in modern literature and history.
Baidu Encyclopedia-Falling Flowers Intentionally, Flowing Water Ruthlessly